The invention relates to a method, a device and a powertrain for operating a hybrid vehicle.
Hybrid vehicles already on the market are characterized in that said vehicles, on the one hand, have a plurality of drive assemblies and, on the other hand, comprise a control system for implementing an operating strategy, which selects a specific type of operation of the two drive assemblies in dependence on the current ambient conditions. The hybrid vehicle can thus be driven alone by one of the drive assemblies or jointly by both of the assemblies. There are also types of operation which are selected on the basis of energy-related or dynamic advantages. Energy-related advantages can, for example, be achieved with a type of operation in which an internal combustion engine is operated at an optimal operating point while an electric machine is not being used to drive the vehicle. During this time period, the electric machine is operated as a generator for generating electrical energy and charging a battery or, respectively, providing electrical loads with electrical energy. Dynamic advantages are, for example, achieved in a type of operation in which the right and left wheel of an axle of a vehicle are driven with different torques or even with opposite torques in vehicles having individually driven wheels on a single axle. Using this type of operation, the handling of the vehicle can be significantly improved when traversing curves, i.e. when accelerating about the vertical axis.
As mentioned above, the different types of operation are selected as a function of different ambient conditions. The German patent application DE 10 2008 001 669 A1 thus discloses that, when the fuel tank level is low, a vehicle is switched over into a type of operation which enables the hybrid vehicle to have a maximum range at a minimum amount of fuel consumption. In addition, the German patent application DE 199 02 949 discloses how to optimize the operation of an internal combustion engine when the tank fill level is low. Methods, which are required to this end, for determining a tank fill level or for detecting a tank that is almost empty are sufficiently known from the German patent application DE 10 2007 030 992 or from the German patent application DE 196 494 84.
If a vehicle is operated, in the fuel tank of which there is no longer much fuel, the probability increases that a drive assembly operated with fuel unexpectedly malfunctions due to the fuel tank approaching empty or being empty. An unexpected failure of a drive assembly leads to an unexpected drop in the driving power or the driving torque of the drive assembly. This can lead to an unexpected and thus critical positive or negative acceleration of the drive wheel and consequently of the vehicle; and said failure can thus negatively impact the handling of the vehicle. Depending on the type of operation in which a hybrid vehicle is operated at the moment of the failure of a drive assembly, the loss of the driving power can cause the vehicle to accelerate at unexpected rates which vary in degree and therefore vary in how critical they are to the safety of the vehicle. The loss of the driving power of a drive assembly of a hybrid vehicle can particularly lead to significantly greater accelerations that are critical to the safety of the vehicle than is the case with conventional vehicles which are solely operated with an internal combustion engine. Hence, the security risk during operation of a hybrid vehicle, in the tank of which no longer much fuel is present, can be increased depending on the type of operation being used.
A technical solution is therefore sought to further secure a hybrid vehicle, i.e. to operate said vehicle without an increased security risk, even if no longer much fuel is present in the fuel tank of the hybrid vehicle.